The following principal uses are permitted in any R-S district:

    A.    One-family dwelling, two-family dwelling, multiple dwelling or dwelling group;

    B.    Field crop, orchard, garden;

    C.    One secondary dwelling unit per building site is permitted on parcels zoned for not more than one dwelling and having one but not more than one existing dwelling unit on the parcel and subject to the following requirements: an existing single-family dwelling may be modified to contain a secondary unit if the existing dwelling contains a minimum of one thousand two hundred (1,200) square feet and was constructed at least twelve (12) years prior to the date of application for such secondary unit with an existing two-car garage and a two-car driveway apron. The secondary unit shall have a direct external entry and shall be limited to a maximum of six hundred forty (640) square feet in area. (Prior gen. code § 8-28.2)